The integrin α2β1 (Very late antigen 2; VLA-2) is expressed on a variety of cell types including platelets, vascular endothelial cells, epithelial cells, activated monocytes/macrophages, fibroblasts, leukocytes, lymphocytes, activated neutrophils and mast cells. (Hemler, Annu Rev Immunol 8:365:365-400 (1999); Wu and Santoro, Dev. Dyn. 206:169-171 (1994); Edelson et. al., Blood. 103(6):2214-20 (2004); Dickeson et al, Cell Adhesion and Communication. 5: 273-281 (1998)). The most typical ligands for α2β1 include collagen and laminin, both of which are found in extracellular matrix. Typically the I-domain of the α2 integrin binds to collagen in a divalent-cation dependent manner whereas the same domain binds to laminin through both divalent-cation dependent and independent mechanisms. (Dickeson et al, Cell Adhesion and Communication. 5: 273-281 (1998)) The specificity of the α2β1 integrin varies with cell type and serves as a collagen and/or laminin receptor for particular cell types, for example α2β1 integrin is known as a collagen receptor for platelets and a laminin receptor for endothelial cells. (Dickeson et al, J Biol. Chem. 272: 7661-7668 (1997)) Echovirus-1, decorin, E-cadherin, matrix metalloproteinase I (MMP-I), endorepellin and multiple collectins and the C1q complement protein are also ligands for α2β1 integrin. (Edelson et al., Blood 107(1): 143-50 (2006)) The α2β1 integrin has been implicated in several biological and pathological processes including collagen-induced platelet aggregation, cell migration on collagen, cell-dependent reorganization of collagen fibers as well as collagen-dependent cellular responses that result in increases in cytokine expression and proliferation, (Gendron, J. Biol. Chem. 278:48633-48643 (2003); Andreasen et al., J. Immunol. 171:2804-2811 (2003); Rao et al., J. Immunol. 165(9):4935-40 (2000)), aspects of T-cell, mast cell, and neutrophil function (Chan et. al., J. Immunol. 147:398-404 (1991); Dustin and de Fougerolles, Curr Opin Immunol 13:286-290 (2001), Edelson et. al., Blood. 103(6):2214-20 (2004), Werr et al., Blood 95:1804-1809 (2000), aspects of delayed type hyersensitivity contact hypersensitivity and collagen-induced arthritis (de Fougerolles et. al., J. Clin. Invest. 105:721-720 (2000); Kriegelstein et al., J. Clin. Invest. 110(12):1773-82 (2002)), mammary gland ductal morphogenesis (Keely et. al., J. Cell Sci. 108:595-607 (1995); Zutter et al., Am. J. Pathol. 155(3):927-940 (1995)), epidermal wound healing (Pilcher et. al., J. Biol. Chem. 272:181457-54 (1997)), and processes associated with VEGF-induced angiogenesis (Senger et al., Am. J. Pathol. 160(1):195-204 (2002)).
Integrins are heterodimers comprised of one α and one β subunit, and comprise a large family of cell surface proteins that mediate cell adhesion to extracellular matrix (ECM) as well as plasma proteins and are central to some types of cell-cell interactions. Integrins interact with ECM components through their extracellular domains. (Pozzi & Zent, Exp Nephrol. 94:77-84 (2003)) Upon binding to ligands, integrins transduce intracellular signals to the cytoskeleton that modify cellular activity in response to these cellular adhesion events, referred to as outside-in signaling (see, e.g., Hemler, Annu Rev Immunol 8:365:365-400 (1999); Hynes, Cell. 110(6):673-87 (2002)). Such signaling can also activate other integrin subtypes expressed on the same cell, referred to as inside-out signaling. Inside-out signaling further occurs via regulatory signals that originate within cell cytoplasm such as a disruption of the clasp between an α and β subunit, which are then transmitted to the external ligand-binding domain of the receptor. Integrins can play important roles in the cell adhesion events that control development, organ morphogenesis, physiology and pathology as well as normal tissue homeostasis and immune and thrombotic responses, and in addition, they serve as environmental sensors for the cell. These proteins are characterized as being in a closed conformation under normal conditions that, upon activation undergo rapid conformational change that exposes the ligand binding site. X-ray crystal structure is a recent tool that has been used in the study of integrin structure and mechanisms of activation. The understanding of integrin structural features facilitates the better understanding of binding sites, differentiated states and their active and inactive formations. In general, the binding site for ligand/counter-receptor for all integrins lies within the α domain and is comprised of a metal ion dependent binding site, referred to as the MIDAS domain (Dembo et al, J Biol.Chem. 274, 32108-32111 (1988); Feuston et al., J. Med. Chem. 46:5316-5325 (2003); Gadek et al., Science 295(5557):1086-9 (2002)); Gurrath et al., Eur. J. Biochem. 210:911-921 (1992)). In the α subunits of the collagen-binding integrins, which include α1, α2, α10 and α11 integrins, the MIDAS site is located within an extra inserted domain at the N-terminus known as the I, A or I/A domain, a feature they share with the α subunits of the leukocyte β2 family of integrins (Randi and Hogg, J Biol Chem. 269: 12395-8 (1994), Larson et al J Cell Biol. 108(2):703-12 (1989), Lee et al., J Biol Chem. 269: 12395-8 (1995); Emsley et al, J. Biol. Chem. 272:28512-28517 (1997) and Cell 100:47-56 (2000)). The I domains are structurally homologous to the A1 domain of von Willebrandt factor, with a Rossman-fold topology of six β-sheet strands surrounded by seven α-helices (Colombatti and Bonaldo, Blood 77(11):2305-15 (1991); Larson et al, J Cell Biol. 108(2):703-712 (1989); Emsley et al, J. Biol. Chem. 272:28512-28517 (1997); Nolte et al; FEBS Letters, 452(3):379-385 (1999)). The collagen-binding integrins have an additional α-helix known as the αC helix (Emsley et al, J. Biol. Chem. 272:28512-28517 (1997) and Cell 100:47-56 (2000); Nolte et al; FEBS Letters, 452(3):379-385 (1999)).
Integrin/ligand interactions can facilitate leukocyte extravasation into inflamed tissues (Jackson et al., J. Med. Chem. 40:3359-3368 (1997); Gadek et al., Science 295(5557):1086-9 (2002), Sircar et al., Bioorg. Med. Chem. 10:2051-2066 (2002)), and play a role in downstream events following the initial extravasation of leukocytes from the circulation into tissues in response to inflammatory stimuli, including migration, recruitment and activation of pro-inflammatory cells at the site of inflammation (Eble J. A., Curr. Phar. Des. 11(7):867-880 (2005)). Some antibodies that block α2β1 integrin were reported to show impact on delayed hypersensitivity responses and efficacy in a murine model of rheumatoid arthritis and a model of inflammatory bowel disease (Kriegelstein et al., J. Clin. Invest. 110(12):1773-82 (2002); de Fougerolles et al., J. Clin. Invest. 105:721-720 (2000) and were reported to attenuate endothelial cell proliferation and migration in vitro (Senger et al., Am. J. Pathol. 160(1):195-204 (2002), suggesting that the blocking of α2β1 integrin might prevent/inhibit abnormal or higher than normal angiogenesis, as observed in various cancers.
Platelets normally circulate in the blood in an inactive resting state, however, they are primed to respond rapidly at sites of injury to a wide variety of agonists. Upon stimulation, they undergo shape changes and become highly reactive with plasma proteins, such as fibrinogen and von Willebrand factor (vWf), other platelets, and the endothelial lining of the vessel wall. These interactions all cooperate to facilitate the rapid formation of a hemostatic fibrin platelet plug (Cramer, 2002 in Hemostasis and Thrombosis, 4th edition). Upon binding ligand, platelet receptors transduce outside-in signal pathways which in turn, trigger inside-out signaling that results in activation of secondary receptors such as the platelet fibrinogen receptor, αIIbβ3 integrin, leading to platelet aggregation. Antibodies or peptide ligand mimetics that bind to or interact with platelet receptors are anticipated to induce a similar signaling cascade leading to platelet activation. Even minor activation of platelets can result in platelet thrombotic responses, thrombocytopenia and bleeding complications.
α2β1 integrin is the only collagen-binding integrin expressed on. platelets and has been implicated to play some role in platelet adhesion to collagen and hemostasis (Gruner et al., Blood 102:4021-4027 (2003); Nieswandt and Watson, Blood 102(2):449-461 (2003); Santoro et al., Thromb. Haemost. 74:813-821 (1995); Siljander et al., Blood 15:1333-1341 (2004); Vanhoorelbeke et al., Curr. Drug Targets Cardiovasc. Haematol. Disord. 3(2):125-40 (2003)). In addition, platelet α2β1 may play a role in the regulation of the size of the platelet aggregate (Siljander et al., Blood 103(4):1333-1341 (2004)).
α2β1 integrin has also been shown as a laminin-binding integrin expressed on endothelial cells (Languino et al., J Cell Bio. 109:2455-2462 (1989)). Endothelial cells are thought to attach to laminin through an integrin-mediated mechanism, however it has been suggested that the α2 I domain may function as a ligand-specific sequence involved in mediating endothelial cell interactions (Bahou et al., Blood. 84(11):3734-3741(1994)).
It is anticipated that a therapeutic antibody that binds α2β1 integrin, including the α2β1 integrin on platelets, could result in bleeding complications. For example, antibodies targeting other platelet receptors such as GPIb (Vanhoorelbeke et al., Curr. Drug Targets Cardiovasc. Haematol. Disord. 3(2):125-40 (2003) or GP IIb/IIIa (Schell et al., Ann. Hematol. 81:76-79 (2002), Nieswandt and Watson, Blood 102(2):449-461 (2003), Merlini et al., Circulation 109:2203-2206 (2004)) have been associated with thrombocytopenia, although the mechanisms behind this are not well understood. It has been hypothesized that binding of an antibody to a platelet receptor can alter its three dimensional structure, and expose normally unexposed epitopes which then leads to platelet elimination (Merlini et al., Circulation 109:2203-2206 (2004). Indeed, the bleeding complications associated with oral doses of GP IIa/IIIb antagonists have been described as the “dark side” of this class of compounds (Bhatt and Topol, Nat. Rev. Drug Discov. 2(1):15-28 (2003)). If α2β1 integrin plays an important role in the movement of leukocytes through inflammatory tissue, it would be desirable to develop therapeutic agents that could target α2β1 for diseases α2β1 integrin-associated disorders and/or cellular processes associated with the disorders, including cancer, inflammatory diseases and autoimmune diseases, if such agents would not activate platelets. Thus, there is a need in the art for the development of compounds capable of targeting α2β1 integrin, such as the α2β1 integrin on leukocytes, which would not be associated with adverse bleeding complications.
The anti-human α2β1 integrin blocking antibody BHA2.1 was first described by Hangan et al., (Cancer Res. 56:3142-3149 (1996)). Other anti-α2β1 integrin antibodies are known and have been used in vitro, such as the commercially available antibodies AK7 (Mazurov et al., Thromb. Haemost. 66(4):494-9 (1991), P1E6 (Wayner et al., J. Cell Biol. 107(5):1881-91 (1988)), 10G11 (Giltay et al., Blood 73(5):1235-41 (1989) and A2-11E10 (Bergelson et al., Cell Adhes. Commun. 2(5):455-64 (1994). Hangan et al., (Cancer Res. 56:3142-3149 (1996)) used the BHA2.1 antibody in vivo to study the effects of blocking α2β1 integrin function on the extravasation of human tumor cells in the liver, and the ability of these tumor cells to develop metastatic foci under antibody treatment. The Ha1/29 antibody (Mendrick and Kelly, Lab Invest. 69(6):690-702 (1993)), specific for rat and murine α2β1 integrin, has been used in vivo to study the upregulation of α2β1 integrin on T cells following LCMV viral activation (Andreasen et al., J. Immunol. 171:2804-2811 (2003)), to study SRBC-induced delayed type hypersensitivity and FITC-induced contact type-hypersensitivity responses and collagen-induced arthritis (de Fougerolles et. al., J. Clin. Invest. 105:721-720 (2000)), to study the role of α2β1 integrin in VEGF regulated angiogenesis (Senger et al., Am. J. Pathol. 160(1):195-204 (2002); Senger et al., PNAS 94(25): 13612-7 (1997)), and to study the role of α2β1 integrin in PMN locomotion in response to platelet activating factor (PAF) (Werr et al., Blood 95:1804-1809 (2000)).
The use of murine monoclonal antibodies, such as those described above, as human therapeutic agents in non-immunocompromized patients has been limited by the robust immune responses directed against administered murine antibodies, particularly in repeated administration. This response cannot only curtail the effective half-life of the murine antibody in circulation but also can lead to profound injection site and/or anaphylactic responses (Shawler et al., J. Immunol. 135(2):1530 (1985)). In addition, the rodent effector functions associated with the constant regions (Fc) are much less effective than their human counterparts when administered to humans, resulting in a loss of potentially desirable complement activation and antibody-dependent, cell-mediated cytotoxicity (ADCC) activity.
Thus, there is a need for the development of antibodies directed against α2β1 integrin, including for treatment of α2β1 integrin-associated disorders, mechanisms, and cellular processes including inflammatory diseases and autoimmune diseases. Moreover, it would be desirable to develop anti-α2β1 integrin antibodies that would not be associated with the development of an anti-murine antibody response in a patient.